(CNN) — A man who was fatally shot by police in Boston on Tuesday was part of a broader terror investigation involving suspected Islamist extremists being tracked by the U.S. Joint Terrorism Task Force, a law enforcement official said.
The suspect belonged to an ad hoc terror network, said a second law enforcement source.
Police and federal agents shot and killed the man after he waved a large military-style knife at officers, authorities said.
The suspect was identified as Usaamah Rahim, according to his brother, Ibrahim Rahim, who posted about the death on social media.
The FBI-led task force had been watching Rahim and two associates believed to be radicalized by ISIS and other extremist influences, according to a law enforcement official. Rahim had been monitored for at least a couple years.
The FBI noted a recent change in the suspect’s behavior, including social media threats against police, which prompted them to try to approach him Tuesday, according to the official.
Ibrahim Rahim, an imam at the Lighthouse Mosque in Oakland, California, wrote on Facebook that his brother was shot while at a bus stop on his way to work.
Ibrahim Rahim said his brother was shot three times in the back during the confrontation. He said his brother was on the phone with their father.
The suspect had been under surveillance by the U.S. Joint Terrorism Task Force, Boston Police Commissioner William Evans told reporters, without elaborating.
Evans said the shooting occurred at about 7 a.m. after officers and FBI agents confronted the unidentified man, who turned around with a large black knife.
The officers retreated and ordered the man to put down the weapon before they opened fire, Evans said.
“Unfortunately, he came at the officers and, you know, they do what they were trained to do and, unfortunately, they had to take a life,” Evans said.
A Boston police officer and a federal agent opened fire on the suspect, Evans said.
The shooting is under investigation.
CNN’s Lorenzo Ferrigno, Camillie Cava and Lawrence Crook contributed to this report.